Generally, it is difficult to measure power usage of computer systems in all the possible scenarios. Many new usage models, applications and data patterns are discovered on regular basis. When a new usage model makes the components of a computer system demand more power than what the power supply can provide, the power supply and accordingly the computer system may fail.
For many computer systems, power specification is based upon analytical models. Such analytical models make certain assumptions on the distribution of power usage by various components of the computer systems. The power specification may be a sum of maximum estimated power consumption of all the components. A power supply may be designed to support this estimated power consumption.
Computer system manufacturers typically provide a power rating or power specification for each of their computer systems. The power specification is based on the maximum estimated power consumption of the components in the computer system. The power specification thus indicates the maximum power that the power supply of the computer system should be able to handle and may be referred to as PMAX. In the process of determining a value of PMAX, computer system designers usually consider a worst-case configuration, which typically means that PMAX is given a value that is based on a computer system fully populated with components.
Furthermore, the value of PMAX may also be based on an assumption that the computer system is configured with the most power hungry components including hardware and software components. For example, a server computer system may be designed to support four processors that may run at between 1.5 GHz and 2.2 GHz, 12 slots of memory, 8 slots for input/output (I/O) adapters and 5 bays for hard drives. The value of PMAX for such a computer system assumes that it is populated with four 2.2 GHz (max power) processors, fully utilized memory slots and I/O slots, and five hard drives. To make the matter worse, computer system designers may include a guard band to reduce probability for system failure. For example, a power supply may have a maximum rating that is a percentage (e.g., 20 percent) higher than the estimated power consumption, resulting in inflated estimated power consumption.
Over designed power supply may drive higher demand on associated infrastructures. This is evident in data centers where computer systems are typically installed on racks. Each rack has limited power and cooling capacity. Many times data center administrators use the inflated estimated power consumption (e.g., based on name plate specification) to determine the number of computer systems that can go on a rack. With the power specification growing in every computer generation, the number of computer systems that can be supported by the rack decreases. As a result, there are more and more empty spaces on the rack. In addition, an over design power supply is unnecessarily expensive.